The invention relates to a continuously operating press for producing boards such as chipboards, fibreboards or the like with two superimposed endless belts comprising heatable plates articulated to one another and rotating about horizontal and parallel shafts whose facing sides are movable in the same direction particularly by drivable feed rollers and each endless belt is enveloped by a further heatable endless steel band. Presses of this general type are known (U.S. Pat. Nos. 2,926,719, 2,981,307 ).
In order to adequately heat the heatable plates and the associated endless steel bands covering these plates, hitherto electrically or steam-heated plates were used which were in contact with the plates of the endless belts to be heated or with the steel bands to be heated. This heating to an adequate temperature of the steel bands and plates with electric power or steam is relatively expensive, even though heating with steam is cheaper than heating with electric power.
The present invention contemplates a more economic manner of heating of the endless belts and steel bands than was hitherto possible. The present invention also contemplates to permit heating in a relatively rapid manner to advantageously above 200.degree. C, as well as to increase the throughput of the continuously operating presses.
To solve the above-discussed problems of the prior art, it is proposed according to a preferred embodiment of the invention to provide both the upper run of the upper endless belt and the lower run of the lower endless belt each with at least one casing provided with at least one suction pipe, wherein are arranged a plurality of flame burners. These flame burners are gas burners, each of which directly heats a portion of the run of the belt, the waste or exhaust gases from the burners being led away in such a way that these gases impinge upon the endless belt also outside the area directly heated by the gas burners. Heating the areas to be heated by gas in this way is not only much cheaper than electrical or steam heating but is also more effective because the waste or exhaust gas obtained is also used for heating the areas to be heated and namely in areas which are located in front of or behind the flame burners. In addition by the conduction of the exhaust gas in the casing a good insulation of the endless belts against cooling is obtained.
In a preferred form of the invention, each gas burner comprises a chamber provided at the top with slots and surrounded by a cooling jacket wherein is arranged a supply pipe with a plurality of openings for a gas -- air mixture. Such cooling air chamber construction associated with the individual gas burners limits or prevents the danger that the burners would be extinguished at the relatively high temperatures of about 600.degree. C. The gas burners preferably are located parallel to one another and extend transverse to the direction of movement of the endless belts. If, for reasons of space, the casing cannot be designed so that it envelops part of the endless belt so that in practice the casing is covered by the endless belt from above, than an adequate utilization of the exhaust gas heat is still achieved although part of the waste gases are led away below the parallel gas burners.
Preferably, however the casing should be designed in such a way that it envelops at least a part of an endless belt and the part of the latter is constructed as a partition optionally with associated sealing means. In this case the corresponding endless belt is acted upon and thereby heated from one side by both the flames of the gas burners and the waste gases removed and on the other side is heated by the waste gases removed. In this last-mentioned arrangement it is preferred that the walls of the casing be arranged spacedly from the portion of the endless belt and to provide sealing means between the side walls of the casing and the portion of the belt which extend to just before the terminal surfaces of the casing, the suction pipe being arranged in immediate proximity of the upper side of the casing. As a result of this construction of the casing, the length of the casing can be made much larger than the width of the endless belt area upon which the gas burners act when viewed in the direction of movement of the belt. After igniting the gas burner the burner exhaust gases heat the belt at points upon which the gas flames have not yet impinged and the portion already heated by the burner flame is further heated by contacting the belt behind the gas burners so that the heat output of the burners is fully used.
To obtain a uniform selectable temperature in the members of the endless belt, the present invention further contemplates that the temperature of the heated plates be measured and the result of the measurements be used via a limit value circuit for switching on and off the burner or burners. In a preferred construction, behind the outlet point of the casing nearest to a deflecting or reversing pulley for a respective endless belt, a contact plate is provided which is pivotally mounted about a horizontal axis and is displaceable perpendicular to the plane of the plate. This contact plate has at least one heat sensor which is connected to a thermometer. Upon reaching a selectable or predetermined temperature in the contact plate, some of the burners, and on reaching a still higher predetermined temperature, further or all of the burners, are disconnected or shut down. Also, on reduction of the detected temperature in the contact plate, part or all of the burners are connected up again. It is preferred to provide at least two burners in each casing to facilitate the above-discussed temperature control and optimize heating.
Since the endless steel bands surrounding the endless belts are also heated by the waste gases from the gas burner, uniform temperatures over the bands are readily obtainable.
It is further proposed by the present invention that, below the upper run of the lower endless steel band, a housing is provided which forms a plurality of zig-zag channels and has partitions to which can be supplied the exhaust gases of the flame burner or burners heating one of the endless belts. This particular arrangement results in a very economical heating of the lower endless steel band, even though the length of this lower band is considerably greater than the upper endless belt to accommodate a material conveying function.
A more uniform temperature of the upper run of the lower endless steel band can be obtained according to the present invention by providing that the waste gas supply connections discharge into the partial chambers of the housing associated with the endless steel band which is located nearest to the enveloped endless belt. Although each partition arranged in the housing can be provided at one of its ends with openings such as holes or slots, it is preferred that the length of each partition be made smaller than the width of the casing because thereby a saving in material and construction time is obtained. In both cases it preferably should be ensured that the waste gases are led through the partial chambers in a zig-zag manner.
These and further objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.